LASER DEVICE WITH LASER STRUCTURES CONNECTED IN SERIES IN AN OPTICAL CAVITY

Abstract: The present invention relates to an edge emitting laser device (10) with at least two laser structures (1, 1b, 1c, 1d) arranged in one and the same optical cavity. The laser structures are substantially arranged in the optical direction of propagation and each laser structure comprises an active region, said active regions being electrically connected in series.

Description: Title: LASER DEVICE WITH LASER STRUCTURES CONNECTED IN SERIES IN AN OPTICAL CAVITY. TECHNICAL FIELD The present invention relates to a laser device which is edge emitting and comprises two or more laser structures in one and the same optical cavity. Devices of this kind can e.g. be used in various light transmission systems such as in fiber optical transmission systems etc. For most diode lasers the threshold current, i.e. the lowest electrical feeding current for which lasering occurs is high whereas the electrical feeding impedance is low. That diode lasers normally has very low electrical input impedances among others makes the construction of broadband feeding circuits under maintenance of a high efficiency difficult. STATE OF THE ART A number of different laser devices with two or more active regions are known. Examples thereof are e.g. so called strip lasers. The reason for them having several active regions is however to increase the power intensity. The active regions therein are furthermore not electrically connected in series. From for example US-A-4 602 370 a laser device is known which has several active layers in a large optical cavity. The different active layers are arranged vertically with intermediate so called distance layers. These active layers are not electrically connected in series and the optical losses are high. In "Intergrated multilayers GaAs lasers separated by tunnel junctions" by J P van der Thiel and W T Tsang, Applied Physics Letter 41 (6), 15 sept., 1982 three so called double heterostructure GaAs-laserdiodes are connected in series through reversed tunnel junctions. Also in this device the active layers are arranged vertically but the arrangement does not form a vertical optical cavity. Since the active layers in such a device are arranged on a relatively large distance from each other, which as such is necessary in this case to avoid optical losses in the contact surfaces or in the tunnel junctions, the different optical light signals from the different lasers will be incoherent since the lasers are not connected to each other. As mentioned above, in well known devices such as for exam¬ ple diodelasers, the electrically input impedance is very low and the threshold current for lasering is high. This has as a consequence considerable difficulties with the con¬ struction of broadband feeding circuits where it is desireable that the efficiency also is high. At the same time impedance adaption through various transmission wires or others gets difficult or impossible. With known devices the dynamic impedance is as low as a few ohms, often only a fraction of one ohm which is very hard to adapt to for example 50 Ω due to the large reflection. In the at the same time and by the same applicant filed copending patent application "Surface emitting laser device with a vertical cavity" a device is described through which a low treshold current and a high impedance is obtained wherein further the resistive, electrical losses which are introduced through the electrical reflection devices, are reduced. In the also simultaneously by the same applicant filed patent application "Optical amplifying device" an amplifying device is described which uses laser structures or active regions connected in series for example in a longitudinal optical cavity according to the present inven¬ tion or a vertical optical cavity in accordance with the abovementioned application. SUMMARY OF THE INVENTION It is an object with the present invention to provide a laser device as initially referred to through which it is possible to lower the treshold current at the same time as the electrical feeding impedance can be increased or to lower the treshold current and increase the voltage drop. A further object with invention is to reduce the treshold current while maintaining the power. It is also an object with invention to enable adaption of the impedance upwards. Furthermore it is an object with invention to provide a device which gives a good and even better amplification without increasing the voltage drop. Another object with invention is to provide a device where no extra conducting junctions are introduced, i.e. more than those which are normally present, i.e. a device where there are no difficul¬ ties with further optical losses. It is also an object with invention to provide a high resistive laser device with a larg voltage drop and a large dynamic impedance. It is also an object with the invention to provide a device which is easy and cheap to fabricate as well as it has to be easy to use and have a broad field of application. These as well as other objects are achieved through a device wherein each laser structure comprises an active region which regions are electrically connected in series and wherein the laser structures are substantially aligned in the optical direction of propagation. According to an advantageous embodiment there are no conduc¬ ting junctions between the laser structures. According to a particular embodiment the optical cavity is substantially longitudinal. Particularly, according to one embodiment, the outer surfaces of the device comprise cleaved, reflective surfaces. Preferably the surfaces of the laser structures which form a boundary towards another laser structure are provided with an antireflective coating or similar. According to a particular embodiment the device comprises a lower electrode and an upper electrode which are divided into a number of laser structures which are sufficiently electrically isolated from each other to be electrically connected in series. Still further the laser structures are arranged so closely to each other that substantially no optical losses are produced. Particularly the device comprises n structures connected in series so that upon injection of a current I 0 in one of the laser structures, n voltage drops in series are obtained. This corresponds substantially to the same power as upon injection of n x I 0 in a conventional, known structure. According to an advantageous embodiment the lower electrode is highly doped. According to a particularly advantageous embodiment the lower electrode of the laser structure is connected to the upper electrode of the adjacent laser structure through a wire, cable or similar. According to an alternate embodiment the wire comprises a structure which is produced through metal deposition and etching. According to one embodiment the laser device comprises four laser structures connected in series. However, of course there can be fewer as well as more laser structures. According to one embodiment the division regions between the laser structures are formed by regions with a high resistivity. The laser structures may according to one embodiment comprise conventional laser structures. According to an advantageous embodiment the device forms a diode laser structure. According to a particular embodiment the laser device may comprise a DBR- laser or a DFB-laser which as such is constructed according to the invention. BRIEF DESCRIPTION OF THE DRAWINGS The invention will in the following be further described in a non-limiting way under reference to the accompanying draw¬ ings wherein: Fig. 1 schematically illustrates a laser device with four active regions connected in series, Fig. 2 illustrates a device with four laser structures within one and the same cavity connected to a voltage source, Fig. 3a illustrates a device in an integrated form, Fig. 3b illustrates an alternate embodiment of the device in an integrated form with a longitudinal connection in series, Fig. 4 shows an example of contacting in the lateral direc¬ tion between different laser structures, Fig. 5a illustrates schematically the application of the invention to a DFB-laser and Fig. 5b illustrates schematically the application of the invention to a DBR-laser. DETAILED DESCRIPTION OF THE INVENTION Fig. 1 illustrates schematically a laser device 10 according to the invention with four laser structures or active regions la, lb, lc, Id connected in series. (The number of structures can of course be lower as well as higher. ) In an ideal case each electron that this injected to an active region generates a photon. Each electron that is injected to the outer contact of the device, i.e. in the shown example the first laser structure la, will give rise to an injected photon in each one of the further active regions, i.e. in each laser structure lb, lc, Id and consequently it would ideally give rise to four photons. The result thereof will be that for the same optical output power, four times lower feeding current and four times higher feeding voltage is required as compared to a conventional arrangement wherein the structures are connected in parallell. Then the feeding impedance will be 16 times higher and the treshold current four times lower. In fig. 2 is in a somewhat more detailed manner a laser device with four laser structures la, lb, lc, Id within one and the same longitudinal optical cavity shown. The outer surfaces 5,5 on the two outermost laser structures la, Id are provided with cleaved reflecting surfaces whereas those surfaces on the laser structures which are arranged adjacent to another laser structure are provided with an antireflective coating 7 or similar. The outer laser structures la, Id may for example be connected to a battery or a voltage source and the bottom electrode 4a, 4b, 4c, 4d of a laser structure is connected through a wire or a conductor 6 to the upper electrode 3b, 3c, 3d of the adjacent laser structure lb, lc, Id, the lower electrode 4b, 4c, 4d in turn being connected to the upper electrode 3c, 3d .... of the next laser structure etc. The laser light is as shown in the figure emitted lengthwise. In fig, 3a is illustrated an example of an integrated form with four upper electrodes 3a, 3b, 3c, 3d and four lower electrodes 4a, 4b, 4c, 4d. The laser structures are arranged on a substrate. The lower electrode layers are highly doped and the regions between the different laser structures la, lb, lc, Id are regions with a high resistivity which for an example can be obtained through ion implantation or etched grooves which are filled with an isolating material. These should have a reflective index which is substantially of the same order of magnitude as the reflective index of the laser material in order to avoid (optical) reflections. The laser waveguide comprises active regions corresponding to each laser structure la, lb, lc, Id under the upper electrodes. In fig. 3b is schematically illustrated an alternative embodiment wherein instead the + and - electrodes alternate as upper and lower electrode respectively. In this case must however the doping be selective, i.e. p-n-junctions have to be switched etc. In fig. 4 is more clearly illustrated how the contacting for example can be effected in the lateral direction wherein an active layer 8 is arranged between the upper and lower electrodes 3a, 3b; 4a, 4b respectively and the laser structure may in other respects form a conventional laser structure. As mentioned above the regions between the laser structures la, lb, lc, Id comprise an isolating material 2 and a "wire" 6 is connected between a bottom contact 4a of the laser structue la and an upper contact 3b of the next following laser structure lb. This wire or conductor 6 may according to an alternative embodiment be replaced by a structure which for example is fabricated through metal deposition and etching etc. Ad¬ vantageously the device comprises a so called monomode structure. The laser structures la, lb, lc, Id have to be arranged so closely to each other that substantially no optical losses occur. A number of different ways of fabri- cation are possible, for example may either different, more or less conventional laser structures be interconnected or two electrodes, an upper electrode and a lower electrode may be split up. These must however be electrically isolated from each other which as mentioned above may be achieved through the isolating material. This can be done in a number of different ways which as such are known. Upon feeding with a current I 0 , 4 voltage drops in series are obtained which gives rise to the same power as if 4 x I 0 is injected to a conventional diode laser with four regions electrically connected in parallell. Through the invention it may be possible to decrease the treshold current for example four times at the same time as the electrical impedance can be increased approximately 16 times. Through the invention no further conducting junctions are introduced and then no further optical losses are produced. The laser device will be more highly resistive than conventional laser devices whereas the voltage drop will be larger. Thereby a dynamic impedance of for example 50 Ω instead of 1 Ω is obtained. The invention can of course be applied to different kinds of DFB-lasers as well as different kinds of DBR-lasers which very schematically is illustrated in figs. 5a and 5b respec¬ tively. In fig. 5a the application of the principle on DFB-lasers is illustrated. The outer laser structures la" and Id" respec¬ tively are provided with an antireflective coating 7 or they have undergone an antireflection treatment on the outer sides and a grating 9 runs through the laser device 40. Fig. 5b illustrates the application on a DBR-laser and 11 illustrate Bragg mirrors. In another aspect the DFB-lasers and the DBR-lasers respectively are formed in a manner known per se. In the shown embodiments the laser device comprises four active regions or four laser structures connected in series. The invention is of course not limited thereto but the number of laser structures connected in series can be fewer as well as more. Also in other aspects the invention is not limited to the shown embodiments but can be freely varied within the scope of the claims.

Claims: CLAIMS 1. Laser device (10;20;30;40;50) with at least two in the same optical cavity arranged laser structures (la,lb,lc,Id,la' ,1b' ,1c' ,Id' ) wherein the laser device is edge emitting, c h a r a c t e r i z e d i n t h a t, the laser structures (la,lb,lc,Id,la' ,1b' ,1c' ,Id' ) are substantially aligned in the optical direction of propaga¬ tion and in that each laser structure (la,lb,lc,ld;la' ,1b' ,1c' ,ld' ) comprises an active region wherein said active regions are electrically connected in series. 2. Device according to claim 1, c h a r a c t e r i z e d i n t h a t, the laser structures (la,lb,lc,Id;la' ,lb' ,lc' ,Id' ) are sufficiently electrically isolated from each other to effec¬ tively be connected electrically in series by contacts or similar. 3. Device to according to claim 2, c h a r a c t e r i z e d i n t h a t, the optical cavity is substantially longitudinal. 4. Device to according to claim 2, c h a r a c t e r i z e d i n t h a t, the outer surfaces (5,5) of the device comprise cleaved, reflecting surfaces. 5. Device according to claim 4, c h a r a c t e r i z e d i n t h a t, those surfaces of each laser structure which form a border to another laser structure are provided with an antreflecting coating (7) or similar. 6. Device according to anyone of the preceding claims, c h a r a c t e r i z e d i n t h a t, it is formed by a lower electrode and an upper electrode between which a laser waveguide (8) with active regions is arranged and in which are divided into a number of from each other electrically isolated laser structures which are electrically connected in series by a wire (6), conductor or similar. 7. Device according to claim 6, c h a r a c t e r i z e d i n t h a t, the laser structures (la,lb,lc,Id,la' ,1b' ,1c' ,Id' ) of the device are arranged so close to each other that practically no optical losses occur. 8. Device according to claim 7, c h a r a c t e r i z e d i n t h a t, this comprises n laser structures and in that upon injection of I 0 to one of the structures, n voltage drops in series are obtained. 9. Device according to anyone of claims 6-8, c h a r a c t e r i z e d i n t h a t, each lower electrode (4a,4b,4c,4d) is highly doped. 10. Device (10;20) according to anyone of claims 6-8, c h a r a c t e r i z e d i n t h a t, the lower electrode (4a,4b,4c,4d) of a laser structure (la,lb,lc,ld) is connected to the upper electrode (3b,3c,3d) of an adjacent laser structure (lb,lc,ld) via a wire or con¬ ductor (6). 11. Device according to claim 10, c h a r a c t e r i z e d i n t h a t, the connection (6) comprises a structure fabricated through metal deposition and etching. 12. Device according to claim 10, c h a r a c t e r i z e d i n t h a t, the border regions or the junctions between the laser struc¬ tures (la,lb,lc,Id,la' ,1b' ,1c' ,Id' ) comprise areas with a high resistivity. 13. Device (10; 20; 30) according to claim 6, c h a r a c t e r i z e d i n t h a t, it comprises four laser structures ( la, lb, lc, Id, la', lb ',1c', Id' ). 14. Device according to anyone of the preceding claims, c h a r a c t e r i z e d i n t h a t, each laser structure comprises a conventional laser struc- ture which is known per se. 15. Device according to anyone of claims 1-3, c h a r a c t e r i z e d i n t h a t, the laser device comprises a DBF-laser. 16. Device according to anyone of claims 1-3, c h a r a c t e r i z e d i n t h a t, the laser device comprises a DBR-laser.

Inventors: NILSSON OLLE, NILSSON, OLLE
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 1994
Main classification: H01S3
IPC codes: H01S3, H01S5